1. Field of the Invention
The present invention relates to exposure control at the time of movie capturing and, more particularly, to exposure control when correcting exposure based on a photometric value.
2. Description of the Related Art
Many digital video cameras have a function of automatically controlling exposure. As an example of the method, a photometric value is acquired from image data, and the aperture, shutter speed, gain, and the like are controlled such that the photometric value reaches a target value equal to a photometric value in correct exposure which is stored in advance. The photometric value is, for example, the average luminance value of the pixels of the whole screen, and serves as the index of brightness of image data.
There are proposed various methods for generating the photometric value. For example, the whole screen is divided into the central portion and the other portion. The average luminance value of pixels of each portion is acquired. A weighted average value is calculated by adding a weight to the value of the central portion. For example, the exposure is controlled such that a value obtained by doubling the photometric values of the pixels of the central portion, adding the photometric values of the pixels of the portion other than the central portion, and dividing the sum by 3 becomes a correct value. In this method, when the main object exists at the central portion, excellent exposure can be obtained for the main object at the central portion.
Japanese Patent Laid-Open No. 2002-118787 proposes a method of obtaining information of a high luminance area in a screen and correcting an exposure control target value while controlling exposure using the photometric value as described above. Since this method suppresses the high luminance area from becoming too bright by lowering the exposure control target value, excellent exposure can be obtained for the main object even when it exists in the high luminance area of the screen.
However, in Japanese Patent Laid-Open No. 2002-118787, the exposure control target value is determined based on the number of areas indicating luminance levels equal to or higher than a predetermined level out of a plurality of areas. The number of areas indicating luminance levels equal to or higher than the predetermined level increases/decreases in accordance with a change of the exposure control target value. Hence, the exposure control target value may be unstable, and it may be impossible to perform excellent exposure control.